How do I delete a question that has already expired?

If you mean in your My Questions list, you can't.

Once resolved, there are only two methods of having them removed:

1. You contact Y/A customer care and convince them to delete the questions. But they only will do that if your questions reveal personal information such as your home address, social security number, and date of birth.

2. The questions are reported for violating the community guidelines.

Edit: If the question is in voting, then you can vote for "no best answer" and if this choice wins, then the question will be automatically deleted.

I don't believe deleting your account will delete the expired questions. I think questions with no BA which expire are deleted automatically, once a BA has been chosen either by you or the voters they can't be deleted by you even if you delete your account.

This is a basic fact that everyone should know before they even turn on a computer. Never type anything on your keyboard that you wouldn't want the whole world knowing.